Constant current circuits are basic circuits which are widely used in various sorts of electronic circuits which require constant currents, for example, a lamp voltage generating circuit and a triangular wave generating circuit. Those constant current circuits are required to supply constant currents to the various sorts of electronic circuits employing the constant current circuits with high precision.
For instance, as shown in FIG. 3, a constant current circuit is composed of transistors 204 and 205 which are p-type MOS transistors, and an n-type depletion transistor 203, and a constant current is supplied to a load 206.
However, the constant current circuit using the depletion transistor as described above has a problem in that current values are varied due to manufacturing variations in manufacturing steps.
To solve the above-mentioned problem, a plurality of sorts of depletion transistors are prepared, trimmings are carried out in order to adjust the current values, and thus, expected current values are obtained (refer to, for example, JP 06-195141 A).
However, when such constant current circuits are employed, proper current values cannot be supplied to circuits such as a regulator, a differential amplifier, a D/A converter, and an A/D converter which are required to be trimmed, unless those constant current circuits have been subjected to trimming adjustment, whereby trimmings for adjusting characteristics of various sorts of electric circuits cannot be carried out.
Thus, a semiconductor circuit using the conventional constant current circuit configuration requires, for example, the following steps:                a. Forming a dummy depletion transistor similar to the constant current circuit in the vicinity of the constant current circuit, and measuring a current value of the depletion transistor.        b. Carrying out a trimming adjustment of the constant current circuit based upon the measured current value.        c. Testing characteristics of various sorts of electric circuits, corresponding to a load.        d. Carrying out trimmings of various sorts of the electric circuits.        e. Performing a shipping check to ship the semiconductor circuit.        
As described above, both the tests of the electric characteristics and the trimming process must be separately carried out with respect to the constant current circuit and each of the various sorts of electric circuits to which the constant current is to be supplied. Accordingly, the above-mentioned 5 steps are required, resulting in such drawbacks that much time and effort are required in manufacturing and that a production cost is increased.
The present invention has been made to in view of the above-mentioned circumstances and therefore has an object to provide a constant current circuit capable of not only acquiring characteristics of current values of various sorts of electric circuits even before a trimming adjustment is carried out, performing the trimming adjustment based upon the acquired characteristics, performing trimming processes to both the constant current circuit and a load collectively, and performing adjustments of the respective characteristics with high precision, but also capable of reducing a total number of manufacturing steps so that the production cost can be lowered, as compared with that of the conventional constant current circuit.